Method for producing hydrogen in a PEM water electrolyser system, PEM water electrolyser cell, stack and system

ABSTRACT

The present invention relates to a method for producing hydrogen in a polymer electrolyte membrane (PEM) water electrolyser cell. A direct electric current is applied to the water electrolyser cell. Water molecules are allowed to diffuse from a cathode compartment through a polymer electrolyte membrane into an anode compartment, to oxidize water molecules at an anode catalyst layer into protons, oxygen and electrons. The protons are allowed to migrate through a polymer electrolyte membrane into the cathode compartment and the protons are reduced at a cathode catalyst layer to produce hydrogen. The cell is supplied with water to the cathode compartment, and humidified air is supplied to the anode compartment. The invention also relates to a polymer electrolyte membrane (PEM) water electrolyser cell, a polymer electrolyte membrane (PEM) water electrolyser stack and a polymer electrolyte membrane (PEM) water electrolyser system.

TECHNICAL FIELD

The invention relates to a method for producing hydrogen using a PEMelectrolyser system and relates to a polymer electrolyte membrane (PEM)water electrolyser system. More specifically, the invention relates to aPEM water electrolysis cell and stack of cells and the operationthereof.

BACKGROUND/PRIOR ART

A water electrolysis cell is an electrochemical device that dissociateswater to produce hydrogen and oxygen gases. An electrolysis cellincludes a cathode, an anode and an electrolyte. The electrolyte ispositioned between the cathode and the anode and transports ions betweenthe electrodes while preventing the transport of electrons. Oneelectrolyte alternative is a polymer electrolyte membrane (PEM), alsocalled a proton exchange membrane. During operation of an electrolysercell, water is oxidized to oxygen gas, protons and electrons at theanode. The protons migrate from the anode to the cathode due to anapplied electric field across the polymer electrolyte membrane. At thecathode, the protons combine with electrons transferred through anexternal circuit to produce hydrogen gas. FIG. 2 shows a schematicdiagram of a membrane electrode assembly (MEA) of a PEM waterelectrolyser cell according to the state of the art and the maintransport phenomena and reactions occurring.

The electrolysis cell consumes water at the anode side and this watermust continuously be supplied to the anode. The water can either besupplied directly to the anode (as shown in FIG. 2) or be supplied tothe cathode and transported through the polymer electrolyte membrane tothe anode. The rate of consumption of water, and thus, the rate ofhydrogen and oxygen generation, is governed by Faraday's law in that anincrease of the current passed through the cell will result in acorresponding increase in the generation of gas and consumption ofwater.

In addition to the water transport, oxygen (O_(2 (diff))) and hydrogen(H_(2 (diff))) are transported through the membrane through adiffusion/convection mechanism due to the partial pressure gradient ofthe gases across the membrane. This gas flux across the membrane and theconsequent mixing of hydrogen in oxygen on the anode and oxygen inhydrogen on the cathode is in state of the art PEM water electrolysersone of the main design and operational constraints: As only a smallamount of hydrogen in oxygen in the anode is needed to form flammableand/or explosive gas mixtures, the hydrogen transported through themembrane will exceed this level if the oxygen production on the anode istoo low (low current densities) or the transport of hydrogen is too high(thin membrane and/or high permeability).

This hydrogen crossover problem is in state of the art PEM waterelectrolysers remedied by using a thick membrane (above 125 μm),preferably made of perfluorosulfonic acid (PSFA) polymers, such asNafion® or Aquvion®, to effectively reduce the hydrogen diffusionthrough the membrane. A hydrogen/oxygen recombination catalyst such asplatinum or palladium may be introduced into the membrane, acting asreaction sites for local recombination of oxygen and hydrogen to water,preventing the diffusing gases to reach the other electrode compartmentand entering the gas phase. However, to have the necessary amount ofrecombination catalyst and time for the recombination reaction to takeplace, it is still necessary to have a significant thickness of themembrane. Thus, state of the art water electrolysers use polymerelectrolyte membranes with thicknesses of 125 microns (Nafion® 115 orequivalent) or higher.

The use of such thick membranes introduces a significant ohmicresistance and consequently a lower efficiency of the electrolyser,especially at current densities above 1 Acm⁻².

Today, water electrolysers are operated with a stack efficiency around65-70% (higher heating value HHV) which results in a demand of about 55kWh of electricity for 1 kg H₂. Of the 55 kWh, about 50 kWh is used bythe electrolysis process and 5 kWh by the balance of plant (circulationand feed water pump, heat exchanger, ion exchanger, gas/waterseparators, valves and sensors). In most water electrolyser systems, thecost of electricity can amount to up to 80% of the cost of the producedhydrogen and an increase in the efficiency of the water electrolyserstack will improve both the overall primary electrical energyconsumption and the total cost of hydrogen.

Current PEM electrolysers are limited in efficiency by mainly twofactors:

-   -   1. The overpotential on the anode    -   2. The ohmic resistance in the polymer membrane.

It is an object of the present invention to provide an improved methodand system for the production of hydrogen by water electrolysis. It isfurther an aim to reduce the energy consumption and consequently reducethe cost of hydrogen produced. Another object of the present inventionis to avoid the formation of flammable or explosive mixtures of oxygenand hydrogen in the electrolyser.

SHORT SUMMARY OF THE INVENTION

In a first aspect of the invention, a method for producing hydrogen in apolymer electrolyte membrane (PEM) water electrolyser cell is provided.The method comprises applying a direct electric current to the waterelectrolyser cell, allowing water molecules from a cathode compartmentto diffuse through a polymer electrolyte membrane into an anodecompartment, oxidizing water molecules at an anode catalyst layer intoprotons, oxygen and electrons, allowing the protons to migrate through apolymer electrolyte membrane into the cathode compartment, reducing theprotons at a cathode catalyst layer to produce hydrogen, supplying waterto the cathode compartment and supplying humidified air to the anodecompartment.

In one embodiment of the invention, the humidified air supplied to theanode compartment has a relative humidity (RH) above 75% RH. Thehumidified air may also be saturated with water. Optionally,supersaturated air is used. The humidified air may be supplied to theanode by use of an air humidifier pump/blower, and distributed throughflow distribution manifolds and via flow patterns on the anode bi-polarplate for optimal gas and water distribution along the active area ofthe anode.

During operation, the pressure on the cathode side of the electrolysercell is preferably controlled to be higher than the pressure on theanode side. Preferably, the pressure on the cathode side is between 0.5bar to 35 bar higher than the pressure in the anode compartment. Duringoperation, the anode compartment is usually operated at a pressureslightly above ambient pressure.

In a second aspect of the invention, a polymer electrolyte membrane(PEM) water electrolyser cell for hydrogen production is provided. ThePEM water electrolyser cell comprises an anode compartment comprising ananode bi-polar plate, an anode metallic porous transport layer, and ananode catalyst layer, a cathode compartment comprising a cathodebi-polar plate, a cathode metallic porous transport layer, and a cathodecatalyst layer, the anode catalyst layer and the cathode catalyst layerare coated on either side of a polymer exchange membrane, wherein thecathode compartment is configured to be supplied with ion exchangedwater through a first set of inlet and outlet flow distributionmanifolds and the cathode bi-polar plate is designed with a first flowfield pattern, and the anode compartment is configured to be suppliedwith humidified air through a second set of inlet and outlet flowdistribution manifolds and the anode bi-polar plate is designed with asecond flow field pattern.

The anode catalyst layer and the cathode catalyst layer may comprisecatalysts in powder form.

The temperature of the supplied air and the relative humidity values areusually at nominal operating temperature of the electrolyser of 50 to90° C.

The polymer electrolyte membrane may have a thickness below 50 microns,preferably in the range of 5 to 49 microns, and most preferred from 10to 35 microns.

In a third aspect of the invention a PEM water electrolyser stackcomprising a plurality of polymer electrolyte membrane waterelectrolyser cells according to the invention, connected in series, isprovided

In a fourth aspect of the invention, a PEM water electrolyser system isprovided. The system comprises the PEM electrolyser stack according tothe invention together with a water and oxygen management system, ahydrogen gas management system, a water input system, mounting andpackaging cabinetry subsystem, a ventilation system, power electronicsand power supply, system controls and instrumentation, and a humidifiedair supply and humidification system.

FIGURES

FIG. 1 is a schematic diagram of an electrolyser cell constructed to beoperated with supply of humidified air on the anode and liquid water onthe cathode.

FIG. 2 is a schematic diagram of a membrane electrode assembly, MEA,according to the state of the art.

FIG. 3 is a schematic diagram of a membrane electrode assembly, MEA,according to the invention.

FIG. 4 is a schematic diagram of a PEM water electrolyser systemaccording to the invention.

FIG. 5 is a diagram showing cell voltage, current density and anode sidegas composition during an electrolyser test.

FIG. 6 is a diagram showing cell voltage at different operatingconditions (proportional to energy consumption).

DETAILED DESCRIPTION

The objects and features of the invention can be better understood withreference to the drawings described below.

FIG. 1 is a schematic diagram of an electrolyser cell constructed to beoperated with supply of humidified air on the anode and liquid water onthe cathode.

The electrolyser cell comprises an anode compartment having an anodebi-polar plate (1 a), an anode metallic porous transport layer (2 a),and an anode catalyst layer (3) coated on top of a thin polymerelectrolyte membrane (4). The cathode compartment comprises a cathodecatalyst layer (5) coated on top of the polymer electrolyte membrane(4), a cathode metallic porous transport layer (2 b) and a cathodemetallic bi-polar plate (1 b). The anode bi-polar plate (1 a) is made ofa metallic material with high corrosion resistance and high electricalconductivity. In addition, the anode bi-polar plate (1 a) is designedwith a flow field pattern (6) and corresponding inlet (7) and outlet (8)flow distribution manifolds for optimal gas and water distribution alongthe active area of the electrolyser. Both the anode bi-polar plate (1 a)and the anode metallic porous transport layer (2 a) are optimised tominimise electrical contact resistances in the electrolyser. The anodemetallic porous transport layer (2 a) is made of a highly corrosionresistant and highly electronic conductive porous material that enablesthe diffusion of humidified air into the anode catalyst layer (3). Theanode catalysts layer (3) comprises a catalyst that is highly efficientfor the oxygen evolution reaction and a proton conductive polymer thatallows for the migration of protons out and water into the anodecatalyst layer (3). The cathode metallic bi-polar plate (1 b) is alsomade of a metallic material with high corrosion resistance and highelectrical conductivity. The cathode bi-polar plate (1 b) is designedwith a flow field pattern (9), but not necessarily the same as the flowfield pattern (6) of the anode bi-polar plate (1 a), a correspondinginlet (10) and outlet (11) flow distribution manifolds for optimal waterand gas distribution along the active area of the electrolyser device,but not necessarily the same as (7) and (8) on the anode side. Thecathode metallic porous transport layer (2 b) is made of a highlycorrosion resistant and highly electronic conductive porous materialthat enables the transport of water and hydrogen in and out of thecathode catalyst layer (5). The cathode catalysts layer (5) comprises acatalyst that is highly efficient for the hydrogen evolution reactionand a proton conductive polymer that allows for the migration of protonsin and water out of the cathode catalysts layer (5).

FIG. 3 shows a schematic diagram of a membrane electrode assembly (MEA)of the PEM water electrolyser cell according to the invention and themain transport phenomena and reactions occurring.

During operation, ion exchanged water (H₂O(I)) is introduced to thecathode compartment of the cell through a stack inlet port, an internalmanifold and a flow field pattern on the cathode bi-polar plate.Humidified air is supplied to the anode compartment through an anodeinlet port, an internal manifold and a flow field pattern on the anodebi-polar plate. A portion of the water on the cathode is absorbed by thepolymer electrolyte membrane and moves to the anode through a combineddiffusion/convection mechanism (H₂O(diff)). Water reacts on the anodeand is converted to oxygen gas, protons and electrons according toequation (1):H₂O→2e ⁻+2H⁺+½O₂  (1)

The protons migrate through the polymer electrolyte membrane from theanode side to the cathode side and by a phenomenon known aselectroosmotic drag, carrying a significant portion of liquid water(H₂O(drag)) from the anode side to the cathode side of the membrane. Atthe cathode, the protons combine with electrons transferred through anexternal circuit to produce hydrogen gas according to equation (2).2e ⁻+2H⁺→H₂  (2)

Any excess water in the anode compartment exits the cell together withair, produced oxygen gas, water vapour ((H₂O(g)) and small amounts ofhydrogen gas. Hydrogen gas produced on the cathode side exits the celltogether with the excess water and traces of oxygen.

The rate of oxygen generation at the anode and the rate of hydrogengeneration at the cathode in the electrolysis cell are governed byFaraday's law in that an increase in the applied cell current willincrease the rate of consumption of water at the anode, and thus, therates of gas generation on both the anode and cathode.

In order to maintain an increased hydrogen generation at a givenelectrode area and stack size, the anode must be supplied withsufficient water.

Continuous operation of the electrolysis cell requires water transportfrom cathode to anode where it is consumed in the oxygen evolutionreaction. In addition to this consumption, other mechanisms also removewater from the anode. Firstly, an effect known as electroosmotic dragdepletes the anode of water, as the protons moving through the membranewill drag an amount of water molecules with them. In Nafion® membranesfor example, the electroosmotic drag can be up to about three moleculesof water per proton.

The anode gas phase in the cell will be undersaturated with water vapourdue to the additional oxygen gas produced at the anode. Liquid water inthe anode will therefore evaporate and leave the anode with the exitinggas and be replenished by water from the membrane.

The diffusion of water through the membrane is proportional to thegradient of the activity of water in the membrane and the diffusioncoefficient of water in the membrane, also known as Ficks law.

The gradient of the activity in the membrane is inversely proportionalto the thickness of the membrane in that a decrease of the membranethickness increases the activity gradient.

In the present invention, the thickness of the PEM membrane may be lessthan 50 microns, preferably from 5 to 49 microns, even more preferred inthe range 10 to 35 microns. Using a thin membrane as described above inthe electrolysis cell, increases the water transport from the cathode tothe anode, resulting in a larger limiting current density, and thus, anincreased hydrogen and oxygen gas generation at a given cell and stacksize.

The activity of water on the cathode is proportional to the pressure onthe cathode. In one embodiment, the pressure of the cathode of theelectrolyser cell during operation is controlled to be higher than thepressure on the anode. This pressure differential will “push” water fromthe cathode to the anode, and thus, improves the water transport fromcathode to anode and results in an increased gas production rate at thesame electrode size. The pressure on the anode is typically slightlyabove ambient pressure in order to overcome the pressure drop of flowinghumidified air through the anode compartment. In one embodiment, thepressure difference between cathode and anode is between 0.5 bar and 35bar, in another embodiment the pressure difference is between 1 bar and20 bar.

The use of a thin membrane as described above, which is significantlythinner than the membranes used in state of the art electrolysis cells,will reduce the ohmic resistance of the electrolysis cell and therebyreduce the energy consumption of the process as much as 15-20%, andthus, reduce the need for external cooling of the electrolysis cell. Athinner membrane will also increase the flux of hydrogen from thecathode to the anode and oxygen from anode to cathode. In a conventionalelectrolysis cell with only water feed on the anode, the increasedhydrogen flux will lead to an increased risk of formation of explosiveor flammable gas mixtures in the anode compartment over a wideroperating range of the electrolysis cell. This invention is mitigatingthis risk by combining the use of a thin membrane with supply ofhumidified air to the anode. The supply of humidified air to the anodewill effectively dilute the hydrogen transported from the cathodethrough the membrane to levels far below the lower explosion limit (LEL)of hydrogen-air mixtures of about 4 mol-%, and thus, removing the riskof the formation of flammable or explosive gas mixtures in the completeoperating range of the electrolysis cell.

Operation of the electrolyser cell may cause degradation of the polymerelectrolyte membrane for example by a free radical attack process. Thisdegradation process is typically highest in the membrane region close tothe cathode due to formation of hydrogen peroxide and free radicals asbiproducts of the reduction of oxygen at the cathode. The rate offormation of free radicals, and consequently, the concentration of thesein the membrane is directly related to the flux of oxygen through themembrane from the anode to the cathode. This flux is a combination ofdiffusion in the polymer phase and diffusion/convection in the waterphase in the membrane. The diffusion rate is generally directlyproportional to the partial pressure of oxygen in the anode (pO₂) andthe convection rate is proportional to the water flux through themembrane.

In one embodiment, as the electrolysis cell is fed with humidified airon the anode, the combination of nitrogen and water vapour in the airprovides a much lower pO₂ than in a conventional PEM electrolysis cell.In addition, the net water flux is from the cathode to the anode asopposed to a conventional PEM electrolysis cell where the net H₂O fluxis from the anode to the cathode (see FIGS. 2 and 3). Thus, anelectrolyser cell operated with humidified air feed on the anode andwater feed on the cathode will have a significantly lower formation offree radicals and a lower membrane degradation rate than conventionalPEM electrolysers.

FIG. 4 shows a schematic diagram of a PEM water electrolyser systemaccording to the present invention. In this system, air is supplied viaa blower or compressor (12) to an air humidifier (13) configured toachieve a controlled humidification level of the air supplied to theanode side of the cells in a PEM electrolyser stack (14). The airhumidifier can be selected from a range of alternatives, such as anenthalpy wheel, membrane humidifier, water atomizer, spray tower orbubble humidifier.

The electrolyser stack (14) is configured to supply the humidified airto each electrolyser cell so that the humidified air is distributedevenly over the surface of the anode electrode so as to dilute hydrogengas permeating from the cathode to a level below 1 volume %. Inaddition, the electrolyser stack is configured to supply liquid water tothe cathode compartment of each electrolyser cell. This combination isvital to secure the necessary water needed for the oxygen evolutionreaction on the anode and to ensure a high water content in the membraneto retain a high proton conductivity. Ion exchanged water is suppliedfrom a water purification device (19). Hydrogen produced exits the PEMwater electrolyser stack (14) together with water. Hydrogen and waterare separated in a hydrogen/water separator (15). The hydrogen flowsthrough a deoxidizer/dryer (16). The separated water is recycled to thewater purification unit (19) and into the PEM water electrolyser stack(14). A circulation pump (17) and a heat exchanger (18) may be includedin the circulation line.

Experiment

An experiment was performed using a MEA based on a Nafion® 212 membrane(50 micron thickness) and mounted in a 25 cm² electrolyser test cell.The test cell was connected to a PEM electrolyser test station fromGreenlight Technologies. During the first two hours of the experiment,the cell was operated at 60° C. in conventional mode at 1 Acm⁻² withwater circulation on the anode and cathode. The concentration ofhydrogen in oxygen was continuously monitored and showed a steady statevalue of about 2 vol % at the anode side. The concentration of hydrogenin oxygen as a function of time is shown in FIG. 5. After two hours, theoperation was changed and 9 l min⁻¹ of humidified air (100% RH at 60°C.) was supplied to the anode while liquid water was supplied to thecathode. The hydrogen concentration in the outgoing gas from the anodeimmediately drops to undetectable (below 0.1%) levels while the cellvoltage and current of the electrolyser is constant.

After 5 hours, the effect of current density was investigated. Theseresults are also shown in FIG. 5. The current density was varied from0.01 to 2 Acm⁻² and no detectable amounts of hydrogen in the outgoinganode gas was detected. As a comparison, the cell was turned back toconventional operation with water on both anode and cathode and thehydrogen concentration quickly increased to about 2 vol % or higher (atlow current density).

After eight hours of operation, the cell was shut down and theexperiment ended. This experiment clearly demonstrates that a PEMelectrolyser with a thin membrane can operate with only humidified airsupplied to the anode inlet with the same performance as a cell suppliedwith liquid water on the anode, but with significantly lower hydrogenconcentrations in the produced gas on the anode. It is possible tomaintain a high current density when water is supplied to the cathode,and humidified air is supplied to the anode, and the hydrogenconcentration is low on the anode side. The inventive method enablessecure operation combined with high efficiency, and thus, loweroperational and equipment costs.

FIG. 6 shows the difference in energy consumption between the use of athick membrane and a thin membrane at different operating conditions.The lines A-D show the effect of different conditions:

A: Thick membrane (125 microns), commercial water electrolyserequivalent. Water on cathode and anode. Inefficient, but SAFE operation(low H₂ concentration at anode)

B: Thin membrane (27.5 microns). Water on cathode and anode: Veryefficient, but UNSAFE operation (very high 3.5 vol % H₂ concentration atanode due to thin membrane)

C: Thin membrane (30 microns). Water on cathode and humidified air onanode: Efficient and SAFE operation (low H₂ concentration (notdetectable) at anode due to dilution. Voltage increase at high currentdue to drier anode)

D: Thin membrane (30 microns). Water with higher pressure on cathode andhumidified air on anode: Very efficient and SAFE operation (Low H₂concentration (not detectable) due to dilution and improved efficiencydue to more water pushed from cathode to anode by higher cathodepressure)

When using water on both the anode and the cathode (state of the art)and a thick membrane, line A, safe operation is obtained, but theprocess is not very efficient. Using a thin membrane and water on bothcathode and anode side, line B, is very efficient, but not safe, as theconcentration of hydrogen increases to more than 3 vol % H₂ in O₂. Whenusing the thin membrane, the energy consumption decreases with around20%.

By operating the cell according to the invention (lines C and D), theconcentration of H₂ is maintained at a low level, i.e. below 0.5%, andthe cell may be operated with a higher current density and lower energyconsumption compared with the state of the art.

The invention claimed is:
 1. A method for producing hydrogen in apolymer electrolyte membrane (PEM) water electrolyser cell, the methodcomprising: applying a direct electric current to the water electrolysercell; allowing water molecules from a cathode compartment to diffusethrough a polymer electrolyte membrane into an anode compartment;oxidizing water molecules at an anode catalyst layer into protons,oxygen and electrons; allowing the protons to migrate through a polymerelectrolyte membrane into the cathode compartment; the polymerelectrolyte membrane having a thickness below 50 μm; reducing theprotons at a cathode catalyst layer to produce hydrogen; supplyingliquid water to the cathode compartment, and supplying humidified air tothe anode compartment; wherein the anode compartment during operation isoperated at a pressure slightly above ambient pressure and the cathodecompartment is operated at a pressure between 0.5 bar to 35 bar higherthan the pressure of the anode compartment.
 2. The method of claim 1,wherein the humidified air has a relative humidity (RH) above 75% RH ata nominal operating temperature of the electrolyser.
 3. The method ofclaim 1, wherein the humidified air is supersaturated air.
 4. The methodof claim 1, wherein the humidified air is supplied through flowdistribution manifold and via flow field patterns on an anode bi-polarplate.
 5. The method of claim 1, wherein the polymer electrolytemembrane has a thickness in the range of 5 to 49 μm.